The present invention relates generally to the field of load-lifting cranes, and more particularly to crane boom segments for such cranes.
Depending upon the requirements of a lift, the length and column strength of a crane boom may vary. For example, crane boom length depends upon the distance between the crane and the object to be lifted, and upon the distance between the object and the height or location to where the object is to be moved. Additionally, the column strength required of a crane boom increases proportionately with the weight of the load to be lifted.
Since the length required of a crane boom will often vary from lift to lift, crane booms are typically comprised of individual boom segments connected end-to-end. The boom segments are usually formed in specific lengths, e.g., 10, 25 and 50 ft. Thus, the length of a crane boom can be tailored to fit each individual lift.
In contrast to the readily changed length of a crane boom, the column strength of a crane boom is not changed to fit a particular lift. Rather, crane manufacturers typically offer a variety of crane booms with differing column strengths. The load requirements of a lift dictates which variety of crane boom is required.
As previously discussed, the greater the load to be lifted, the greater the column strength required of the crane boom. The column strength of a boom is a well-known function of the cross-sectional area of the material used in its chord members, the strength of that material and the distance those chord members are from the center-line of the column. One method of increasing the column strength of a boom without increasing the amount of material used in the chords (and hence the weight of the boom) is to space the chords further from the center-line of the boom. This, however, increases the overall width and/or height of the boom section. Transportability problems arise with crane boom sections of large dimension. If any of the dimensions is too large, the crane boom segments cannot be transported along highways, railways and the like due to size restrictions. Thus, difficulties arise in moving crane booms of large dimensions to job sites. Because of these restrictions, it was previously thought that a crane boom 10 feet 10 inches wide and 9 feet 7 inches high was about as large of a boom as would be acceptable for highway transport.
Three approaches have been used to overcome this problem, all of which have distinct disadvantages. The first approach, practiced by Neil F. Lampson, Inc., consists of transporting the individual parts of each crane boom segment to the specific jobsite and constructing the crane boom segments on-site. Specifically, Lampson positions the chord members of the crane boom segments on-site and then bolts the lacing members for each crane boom segment to the chord members thereof. After the crane boom segments are constructed, they are connected end-to-end to form the crane boom. This approach requires time-consuming and labor-intensive construction.
The second approach, believed to be used by Mannesmann Demag Baumaschinen, is to utilize crane boom segments of a sufficiently small dimension to allow them to be transportable, but to form the chord members with very thick walls. While the small overall dimension allows the crane boom segments to be transported easily, the additional weight causes the crane boom to be heavier and thus a less efficient column member.
The last approach is to transport a sufficient number of crane boom segments to the jobsite such that two or more crane booms may be formed. The separate crane booms are then used side-by-side, in conjunction with one another, to complete the required task. This approach has the disadvantage of requiring the assembly of multiple crane booms, and further of adapting the crane booms so that they can be used as one unit instead of separate units.
The present invention solves the transportability problem of crane boom segments of large dimension without the undesirable use of larger and heavier chord members or the need for difficult and time-consuming construction of individual crane boom segments or crane booms on the job site.